In recent years, in order to protect global environment, weight reduction of automobile bodies is demanded as a measure to decrease the amount of CO2 discharged from automobiles, and increases in the strength of steel materials for automobiles are required. This is because increase in the strength of steel material will allow decrease of the thickness of steel material for automobiles. On the other hand, there are increased demands for safety of automobiles in collisions. Accordingly, there is need for not only simply increasing the strength of steel material, but also developing a steel material having improved impact resistance upon collision during driving.
In such cases, since each portion of a steel material for an automobile is subjected to deformation at a high strain rate of at least several tens of seconds−1 in collision, a high strength steel material having improved dynamic strength properties is required. As such a high strength steel material, there are known a low alloy TRIP steel having a high static-dynamic difference (difference between static strength and dynamic strength), and a high-strength multi-phase structure steel material such as a multi-phase structure steel having a second phase primarily including martensite.
Regarding the low-alloy TRIP steel, for example, Patent Document 1 discloses a strain induced transformation-type high-strength steel sheet (TRIP steel sheet) having improved dynamic deformation properties and for absorbing automobile collision energy.
On the other hand, regarding the multi-phase structure steel sheet having a second phase primarily including martensite, the followings are proposed.
Patent Document 2 discloses a high-strength steel sheet having improved balance of strength and ductility and a static-dynamic difference of at least 170 MPa, wherein the steel sheet is made up of fine ferrite grains, and the average grain diameter ds of nano crystal grains having a grain diameter of at most 1.2 μm and an average grain diameter dL of micro crystal grains having a grain diameter of more than 1.2 μm satisfy a relationship of dL/ds≧3.
Patent Document 3 discloses a steel sheet having a high static-dynamic ratio, wherein the steel sheet has a dual-phase structure of martensite having an average grain diameter of at most 3 μm and ferrite having an average grain diameter of at most 5 μm.
Patent Document 4 discloses a cold-rolled steel sheet having improved impact absorption properties, wherein the steel sheet contains at least 75% of ferrite phase having an average grain diameter of at most 3.5 μm, the remainder being tempered martensite.
Patent Document 5 discloses a cold-rolled steel sheet having a static-dynamic difference of at least 60 MPa at a strain rate of 5×102 to 5×103/s, wherein the steel sheet is made to have a dual-phase structure of ferrite and martensite by pre-straining.
Patent Document 6 discloses a high-strength hot-rolled steel sheet having improved impact resistant properties, wherein the steel sheet is made up only of at least 85% of bainite and a hard phase such as martensite.